<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script>
        function start(a) {
            num1=document.getElementById("num1").value;
            num2=document.getElementById("num2").value;
            var num3=0;
            if (a=="+"){
                num3=Number(num1)+Number(num2);
            }
            if (a=="-"){
                num3=Number(num1)-Number(num2);
            }
            if (a=="*"){
                num3=Number(num1)*Number(num2);
            }
            if (a=="/"){
                num3=Number(num1)/Number(num2);
            }
            document.getElementById("result").value=num3;
        }
    </script>
</head>
<body>
<input type="text" id="num1">
<input type="text" id="num2">
<input type="button" value="+" onclick="start(value)">
<input type="button" value="-" onclick="start(value)">
<input type="button" value="*" onclick="start(value)">
<input type="button" value="/" onclick="start(value)">
<input type="text" id="result">

</body>
</html>